BRUSSELS — Defense Secretary Robert M. Gates on Wednesday urged more NATO allies to join the air campaign against Libya, arguing that it was putting a strain on the seven members of the 28-nation alliance that are carrying the burden in a conflict that shows few signs of ending soon, U.S. officials said.

In a sign of the growing strain that the 3-month-old operation is putting on the North Atlantic Treaty Organization, Gates took the unusual step of naming five alliance members with limited or no role in the operation that he said should provide strike aircraft to hit ground targets in Libya or other capabilities, according to a senior U.S. official.

Gates made the comments at a closed-door meeting of NATO ministers that endorsed continuing the air operation for another three months.

Although NATO officials hoped the meeting would send a message that the alliance is united and determined to continue the war until Libyan leader Moammar Gadhafi gives up power, the signs of divisions within the alliance raised questions about its staying power if Gadhafi continues to hold, despite the increasing pounding Tripoli and other cities have taken in recent weeks.

Gates, who is retiring at the end of the month, called on Germany and Poland, which have refused to participate in the Libya campaign, to contribute. He also urged Spain, Turkey and the Netherlands, which are participating but not in airstrikes, to step up their role, according to officials familiar with the discussion.

Calling the conflict a "war of attrition" and a "psychological war," the senior U.S. official said that "crews are getting tired" and that "the stress on aircraft is significant." With only a handful of alliance members participating, he added, "it doesn't mean they can't continue the operation; they will, but it's stressful."

Only France, Britain and five other Western members are conducting airstrikes against ground targets in Libya, and as operations continue the strain on their armed forces has grown severe, officials said. At the meeting, Norway's representative said his country was reviewing whether its air force could continue its current level of participation, the U.S. official said.

In addition, the official said, the air campaign is straining the military budgets of those conducting airstrikes because they had not planned for the cost of such a long campaign and have to replenish their munitions stockpiles.

Other countries involved in the air campaign include the United States, Canada, Italy, Denmark and Belgium. After leading the initial air assault on Libya in the first weeks of the war, the United States scaled back its involvement and is mostly providing aerial refueling, surveillance and other support functions, as well as several Predator drones, which are being used in airstrikes.

None of the countries named by Gates committed to increase their participation. On Tuesday, President Barack Obama met with German Chancellor Angela Merkel and at least publicly did not put pressure on her nation to participate in the air campaign.

PAKISTAN: At least 20 suspected militants were killed in Pakistan's northwestern tribal region Wednesday in what were described as a pair of American drone strikes, local news media reported.

Eds: New version. An interactive about Libya is in the file international/gadhafi. AP Video. With AP Photos.

By ROBERT BURNS and BRADLEY KLAPPER

Associated Press

BRUSSELS (AP) — The Obama administration challenged five key military allies Wednesday to take on a greater share of the NATO-led air campaign against Moammar Gadhafi's forces, illustrating the strains of a three-month intervention in Libya that has no timeframe for an exit.

Defense Secretary Robert Gates delivered the pointed message in a goodbye to his counterparts from NATO ahead of his retirement next month, senior American and British officials said. But none of the nations challenged pledged to do more.

The pressure on Germany, Poland, Spain, Turkey and Netherlands comes as the alliance continues with intensified airstrikes on Libya's capital and only a day after President Barack Obama played down any suggestions of a rift with German Chancellor Angela Merkel over her nation's lack of contribution to the war effort.

"Secretary Gates was very blunt," said Liam Fox, the defense secretary of Britain, which along with France has led the mission to protect Libyan civilians from Gadhafi's troops.

Gates said Spain, Turkey and the Netherlands should enhance their limited participation in noncombat operations by joining in strike missions against ground targets, U.S. officials said on condition of anonymity to discuss internal NATO deliberations. They said Gates pressed Germany and Poland, the two countries not participating at all militarily, to help in some form.

Secretary of State Hillary Rodham Clinton will likely restate Gates' argument Thursday, when NATO nations and Arab governments participating in the air campaign meet in the United Arab Emirates.

Clinton also is expected to tell the gathering that the U.S. will carry on providing military logistics support and emergency backup help for the campaign, but will resist any entreaties to expand its role in other ways. And she will hedge U.S. engagement with a Libyan rebel-affiliated group seeking to set up an alternative government. Clinton has met with its representatives and the U.S. has eased sanctions on Libya to free up money for the would-be civilian government, but has not formally recognized it as legitimate.

State Department spokesman Mark Toner said the U.S. would talk "about ways that we can intensify pressure on Gadhafi," without wanting to wade into military considerations.

Gates, according to officials, said the additional support wasn't needed to continue air operations for another 90 days, which has been welcomed by all 28 NATO countries. But he said it was a necessary element of fairness in an alliance built on the principle of shared burdens.

Eight NATO members are participating in air strikes in Libya: The U.S., Britain, France, Belgium, Canada, Norway, Denmark and Italy. Denmark and Norway in particular are contributing disproportionately, given the size of their militaries, U.S. officials said, and both are feeling the stress on their aircraft and crews as well as a financial strain.

These stresses, combined with the refusal of some alliance members to participate at all in offensive operations, are one reason U.S. officials believe NATO is excessively dependent on the United States for its advanced military power.

Britain echoed the U.S. concerns.

"What we have been trying to say today is that there are other countries that have assets which we could be using, not necessarily ground attack but in support and reconnaissance, in air-to-air refueling and so on," Fox said. "Too many are doing too little. Let's hope they mull over that this evening and that we get further responses in the next few days."

The U.S. and its allies say the upturn in NATO airstrikes is increasingly pinning Gadhafi into a corner, even as troops loyal to the erratic leader of 42 years lashed back with renewed shelling of the western city of Misrata on Wednesday, killing 10 rebel fighters. Yet NATO's strikes on Wednesday — at least four strikes during the day after five before dawn — reinforced the limits to its intervention.

Despite overwhelming aerial power, it has been difficult for the international coalition to end the threat from Gadhafi without overstepping its mission. NATO says it is scrupulously following the U.N. resolution calling for the protection of civilians, and not for the ouster of Gadhafi, who is vowing to fight to the death.

By singling out countries, Gates was putting longstanding allies on the spot at a time when NATO leaders are emphasizing their solidarity in the Libya mission. NATO Secretary General Anders Fogh Rasmussen told reporters Wednesday the alliance is well on its way to fulfilling the mission and paving the way for a post-Moammar Gadhafi period.

But nearly three months after operations started, no one can convincingly say how fast the intervention will be able to end. The opposition holds maybe a third of the inhabitable area of the country, but has struggled to make advances. And as long as Gadhafi's forces remain close to inhabitable areas, the threat to civilians persists.

The U.S. military moved to a secondary role after the initial period of air and naval bombardment that established a no-fly zone over the North African country and opened the door to the NATO-led air campaign.

Obama has declined to put U.S. warplanes back into an offensive role — aside from a relatively small number of planes targeting Libya's air defenses. But a few weeks ago the U.S. provided nine more aerial refueling planes to enable NATO to accelerate its bombing, the U.S. officials said.

The U.S. is contributing about 75 percent of the aerial refueling capacity for the campaign and 70-80 percent of the intelligence, surveillance and reconnaissance capability, they said.

In a separate presentation to Wednesday's meeting, NATO's top commander, U.S. Navy Adm. James Stavridis, said he has enough aircraft and munitions to continue the air campaign for another 90 days, the officials said. Gates' point to the defense ministers was that the alliance must more equitably share the risk and costs associated of engaging in combat — regardless of how long the campaign lasts.

———

Klapper reported from Washington.

Mideast roundup

yemen: The New York Times reported late Wednesday that the Obama administration has intensified the U.S. covert war in Yemen, exploiting a growing power vacuum in the country to strike at militant suspects with armed drones and fighter jets. Also Wednesday, Yemeni opposition leaders and protesters intensified their demands for the creation of a presidential council that would take power from the country's president, Ali Abdullah Saleh.

syria: European nations seeking to increase pressure on President Bashar Assad's regime presented a revised resolution to the United Nations on Wednesday condemning Syria for its deadly crackdown on peaceful protesters.

iran: Iran declared Wednesday that it plans this year to triple production of its most concentrated form of nuclear fuel, expanding its manufacturing efforts to a mountainous, once-secret nuclear site deep underground. Experts worry that the production of more concentrated fuel could bring Iran closer to the ability to rapidly make weapons-grade uranium for a bomb.

tunisia: Tunisia will hold an election Oct. 23, not in July as planned, because conditions are not yet right since the ouster of the country's president, the prime minister said Wednesday.